Method of assembling elements by localized heating

ABSTRACT

The invention relates to a process of assembly of at least two silicon substrates. The method comprises:
         a step of placing in contact a first silicon substrate ( 9 ) with a second silicon substrate ( 10 ), the first and second substrates ( 9, 10 ) being substantially non-transparent for a wavelength λ of laser radiation (R), and   a step of illuminating the first silicon substrate ( 9 ) with a laser beam of wavelength λ to create a fusion path ( 21 ), along the laser beam axis (A 1 -A 2 ), in the thickness of the first substrate ( 9 ) and in all or part of the thickness of the second substrate ( 10 ).       

     The invention is applied to the sealing of cavities and of mechanical or electrical joints situated at the interface of two silicon substrates.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority based on International PatentApplication No. PCT/FR02/00727, entitled “Method For Assembling ElementsBy Localized Heating” by Henri BLANC which claims priority of Frenchapplication no. 01/02884, filed on Mar. 2, 2001, and which was notpublished in English.

TECHNICAL FIELD AND PRIOR ART

The present invention relates to a method of assembly of elements bylocalized heating.

More particularly, the invention relates to a method of assembly bylocalized heating of at least two substantially planar siliconsubstrates of low thickness. The two silicon substrates can be, forexample, plates, laminae, slices, thin films or thin layers.

The invention is applied, for example, to the sealing of cavities and ofmechanical or electrical joints situated at the interface of twosubstrates.

According to the known art, the substantially planar assembly ofelements is formed with or without addition of material.

Assembly without addition of material may be performed by placing twopolished surfaces in contact after chemical preparation of the surfaces,then heating for consolidation. The materials placed in contact may beSi/Si, Si/SiO₂, Si/metal, or metal/metal. This type of assembly is knownto those skilled in the art by the term “wafer bonding”. Another type ofassembly without addition of material is known under the termanodic-type assembly. It then relates to establishing the bond betweentwo materials reacting at the interface under the conjoint effect of anelectric field and of temperature (for example, a Si/Pyrex glass orAl/Pyrex glass interface).

Assembly with the addition of material may be an adhesive type assembly.An adhesive intermediary, for example epoxy resin, may be localizedbetween the elements to be assembled. A metallic type assembly maylikewise be concerned:

-   -   addition of a metal, the alloy of which with the materials to be        assembled has a melting point lower than that of the metal        (e.g., Si/metal/Si),    -   hybridization of elementary chips on the substrate,    -   fusion of metals.

The assembly methods of the prior art have numerous disadvantages.

The chemical preparation of the surfaces is thus prohibited for theassembly of already completed and thus fragile circuits. The same istrue for methods necessitating a rise in temperature of the circuits.The addition of intermediate material is likewise a disadvantage.

SUMMARY OF THE INVENTION

The invention does not have these disadvantages.

In fact, the invention relates to a method of assembly of at least twosilicon substrates. The method comprises:

-   -   a step of placing in contact at least one substantially planar        face of a first silicon substrate with a substantially planar        face of a second silicon substrate so as to constitute an        interface between the first and second substrates, the first and        second substrates being substantially non-transparent for a        laser radiation wavelength λ, and    -   a step of illuminating the first silicon substrate with a laser        beam of wavelength λ to create a fusion path, along the laser        beam axis, in the thickness of the first substrate and in all or        part of the thickness of the second substrate.

According to a supplementary characteristic of the invention, thecreation of the fusion path is accompanied by a reduction of mechanicalstrength of the silicon at the interface between the fusion path and theremainder of the silicon substrate and, on both sides of the fusionpath, over a finite distance, direct sealing of the interface betweenthe two silicon substrates.

According to yet another supplementary characteristic of the invention,the laser beam of wavelength λ is displaced on the surface of the firstsubstrate so as to create successive fusion paths defining a plane. Inthis latter case, according to yet another supplementary characteristicof the invention, the method comprises a step of cutting by cleavagealong at least one plane created by the successive fusion paths.

According to yet another supplementary characteristic of the invention,the laser beam of wavelength λ is displaced on the surface of the firstsubstrate so as to create successive fusion paths defining a non-planarsurface. In this latter case, according to yet another supplementarycharacteristic of the invention, the method comprises a step of cuttingby KOH etching along at least one non-planar surface created bysuccessive fusion paths.

According to another supplementary characteristic of the invention, themethod comprises a step of forming a vacuum between the two faces of thesilicon substrates which are placed in contact.

According to yet another supplementary characteristic of the invention,the laser radiation is infrared radiation of wavelength λ substantiallyequal to 1064 nm, of mean power substantially equal to 12 W, andconstituted by pulses of frequency substantially equal to 3 kHz.

The contacting surfaces to be sealed have very low roughness and goodplanarity. Outside the surfaces to be sealed, the facing surfaces may bestructured and consequently spaced apart. This is the case, for example,in circuits manufactured by micro-technology according to integratedcircuit technology.

The method of assembly acts by direct sealing of the substrates becauseof the quality of contact present at the interface of the substrates.

Advantageously, the zones placed in contact according to the method ofthe invention may define a cleavage path for later cutting.

It is likewise possible to prolong the method according to the inventionitself as far as the cutting of circuits. In the same operation, it isthen possible, for example, to assemble certain elements and to cut offothers from them.

In the two cases, the cutting-off can advantageously be performedwithout the usual protections used in the standard cutting methods(water jet, particle jet, circuit reversal, etc.).

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will becomeapparent on reading an embodiment of the invention with reference to theaccompanying figures, among which:

FIG. 1 shows an arrangement for assembling two planar elements of lowthickness according to the invention;

FIG. 2 shows the action of laser radiation of wavelength λ on a planerelement of low thickness, not transparent to a wavelength λ;

FIG. 3 shows an example of a structure assembled according to theinvention;

FIG. 4 shows the action of a laser beam for the assembly of a structureaccording to FIG. 3;

FIG. 5 shows the energy distribution of a laser beam used for assemblinga structure such as shown in FIG. 3;

FIG. 6 shows the temperature distribution in a band of silicon subjectedto a laser beam whose energy is distributed as shown in FIG. 5;

FIG. 7 shows the distribution of mechanical strength of a band ofamorphous silicon according to the invention.

In all the figures, the same reference numerals denote the sameelements.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows an arrangement for assembling two planar elements of lowthickness according to the invention.

Two planar elements of low thickness 5 and 6, for example, two siliconplates or a substrate supporting a thin layer facing another plate oranother thin layer supported by another substrate, are placed within acavity C and placed in mutual contact. The cavity C is defined by thespace formed between a support 2 and a cover 1. An orifice O connectedto a vacuum pump (not shown) permits evacuation of the cavity C andconsequently a vacuum to be formed between the two faces to be united.Joints 4 permit isolating the volume comprised between the two surfacesto evacuate it so that the two surfaces come into perfect contact.

In the case of non-deformable substrates, the surfaces will have to havequasi-perfect planarity.

In the case of at least one thin, and thus deformable, substrate,planarity over a short distance will be sufficient. A lack of planarityover a long distance may be compensated by a deformation of the thinsubstrate. The cover 1 has as large as possible an opening 3 for aradiation R of wavelength λ from a laser L. The radiation R can thenreach the element 5. The laser L is, for example, a pulsed YAG powerlaser emitting in the infrared. By way of example, without limitation,for two silicon plates 450 μm thick, the wavelength λ of the radiationis equal to 1064 nm, the mean power is equal to 12 W, and the pulsefrequency is 3 kHz. The laser radiation may be controlled, for example,to obtain a beam of diameter between 30 and 50 μm. The mean power at thesurface of the element 5 can then be equal to 8 or 9 W.

The power absorbed by the silicon at the infrared frequency is low. Theresult is melting of the silicon with very little ejected material wherethe laser beam passes over the elements 5 and 6. There is thenamorphization of the silicon over the whole thickness passed over.

FIG. 2 shows the action of a radiation R of diameter D on a thin layerof silicon 7. Material is ejected over a very low depth, while a band ofamorphous silicon 8 is formed on the beam path. To form the band ofamorphous silicon, the displacement speed of the laser beam over thesurface of the layer of silicon may be comprised, for example, between0.5 mm/s and 2 mm/s.

FIG. 3 shows an example of a structure assembled according to theinvention.

The structure S comprises two silicon substrates 9 and 10 fixed togetherby three bands 11, 12, 13 of amorphous silicon. The thickness e1 of thesubstrate 9 is for example equal to 300 μm and the thickness e2 of thesubstrate 10 for example equal to 500 μm. Advantageously, only a portionof the amorphous silicon bands fixes the substrates together. As willbecome apparent from the description hereinafter, fixing the substrateis principally performed by zones situated on both sides of the fusionpath.

The structure S constitutes, for example, a seismic sensor. Thesubstrate 10 then comprises an assembly of active zones or sensors 15,17, 19. Cavities 14, 16, 18 are formed above the respective sensors 15,17, 19. The substrate 9 is a cover intended to protect the sensors fromshocks and dust and permits electrical contacts to be made throughapertures (not shown in the figure).

FIG. 4 shows the action of a laser beam for the assembly of a structureaccording to FIG. 3, and FIGS. 5 and 6 respectively show the curve oflaser beam energy distribution and the curve of temperature distributionin a silicon band during the assembly of the structure according to FIG.3.

As has previously been mentioned, the power absorbed by silicon atinfrared frequencies is low. As a result, very little material 20 isejected when the laser beam passes successively over the siliconsubstrates 9 and 10.

A zone of molten silicon 21 appears over a width d in the silicon. Thewidth d, for example equal to 40 μm, substantially corresponds to thewidth of the laser beam for which the beam energy E is maximal (cf. FIG.5). In the silicon melted by the action of the laser beam, thetemperature varies from substantially 2,600° C. at the level of the axisA1-A2 of the laser beam to substantially 1,400° C. at a distance d/2from the axis A1-A2 (cf. FIG. 6). Boiling of the silicon at the centerof the molten zone 21 advantageously leads to a certain porosity of thiszone. An interface 22 appears between the molten zone 21 and thenon-molten, monocrystalline silicon of the substrates 9 and 10 situatedbeyond the molten zone (clear transition between liquid phase and solidphase).

The temperatures in the silicon decrease rapidly on either side of thezone 21 of width d, going away from the axis A1-A2 of the beam, forexample from substantially 1,400° C. to substantially 400° C. Thesetemperatures are however sufficient to permit a junction between thesubstrates 9 and 10 by direct sealing between the two substrates. Thisjunction by direct sealing takes place, on either side of the zone 21,over the respective finite distances 11 and 12.

After the formation of the bands 11, 12, 13 of amorphous silicon asshown above, the assembled structure of FIG. 3 advantageously remainsintegral, that is, manipulable without risk of fracturing. It is thenpossible to pursue different types of technological steps which do notlead to high mechanical stresses. It is likewise possible to cut thestructure along the interfaces 22.

As shown in FIG. 7, the mechanical strength which unites the substrates9 and 10 is low along the interfaces 22 and relatively high in the zonesof widths 11 and 12 situated beyond the fusion band.

Cutting by cleavage along the interfaces is possible when the interfaces22 define cleavage planes. When the interfaces 22 are not planes, forexample when they define closed curves, cutting may be performed by KOHetching (KOH for potassium hydroxide). Advantageously, KOH etching forseveral minutes may be enough to dissolve an amorphous silicon band 30μm wide over 500 μm depth such as a band according to the invention (byway of comparison, several hours are necessary for dissolving athickness of untreated silicon of the same thickness).

The KOH etching can likewise by shortened by treating the substrate inthe following manner: immersion in isopropanol, then in ethanol, then indeionized water, then finally in KOH. KOH etching may likewise beassisted by ultrasound.

1. Method of assembly of at least two silicon substrates, comprising: astep of placing in contact at least one substantially planar face of afirst silicon substrate with a substantially planar face of a secondsilicon substrate so as to constitute an interface between the first andsecond substrates, the first and second substrates being substantiallynon-transparent for a wavelength λ of laser radiation, and a step ofilluminating the first silicon substrate with a laser beam of wavelengthλ to create a fusion path, along the laser beam axis, in the thicknessof the first substrate and in all or part of the thickness of the secondsubstrate, wherein the creation of the fusion path is accompanied by areduction of mechanical strength of the silicon at the interface betweenthe fusion path and the remainder of the silicon substrate and, oneither side of the fusion path, over a finite distance, a direct sealingof the interface between the two silicon substrates.
 2. Method accordingto claim 1, wherein the laser beam of wavelength λ is displaced over thesurface of the first substrate so as to create successive fusion pathsdefining a plane.
 3. Method according to claim 2, comprising a step ofcutting by cleavage along at least one plane created by successivefusion paths.
 4. Method according to claim 1, characterized in that thelaser beam of wavelength λ is displaced over the surface of the firstsubstrate so as to create successive fusion paths defining a non-planarsurface.
 5. Method according to claim 4, comprising a step of cutting byKOH etching along at least one non-planar surface created by successivefusion paths.
 6. Method according to claim 1, comprising a step offorming a vacuum between the two faces of the silicon substrates whichconstitute the interface.
 7. Method according to claim 1, wherein thelaser radiation is infrared radiation of wavelength λ substantiallyequal to 1064 nm, of average power substantially equal to 12 W, andconstituted by pulses of frequency substantially equal to 3 kHz.